1. Field of the Invention
The present invention relates to the mounting of active optical devices which are electrically driven.
2. Discussion of Prior Art
While the reference to active optical devices which are electronically driven includes devices exhibiting an electro-optic effect, it also encompasses any other active or variable optical device which requires an electrical input, even if the optical effect is not an electro-optic one—for example a magneto-optic effect where the magnetic field is produced by electrical means, or a device in which mechanical movement is produced electrically. Hereafter, all such devices will be termed electro-optic devices.
There has been a requirement for increasing speed of operation of electro-optic devices. At the same time there has also been a requirement for more complex devices, for example spatial light modulators comprising a matrix with a large number of pixels for high resolution of an image, or more accurate signal processing, which in itself leads to an increase in the time needed to address an individual pixel matrix, due both to the increasing complexity of the addressing system and the length of conductors needed to reach pixel, and also an increase in time necessary to address all pixels in the matrix.
In part, improved speed has been facilitated by the discovery of new effects or materials—thus in the field of liquid crystal technology the use of fast switching chiral smectic liquid crystal materials has become increasingly important. Also, as with many electronic devices, improved speed can be obtained by making the devices smaller, to reduce conductor lengths.
One construction of electro-optic spatial modulator device that has become increasingly significant is that using an active backplane. The latter is normally used in conjunction with a spaced opposed front electrode, with electro-optic material in the gap between the backplane and front electrode.
Recently there been developed a novel spatial light modulator in the form of a smectic liquid crystal layer disposed between an active semiconductor backplane and a common front electrode. It was developed in response to a requirement for a fast and, if possible, inexpensive, spatial light modulator comprising a relatively large number of pixels (for example, 320×240 or 640×480) with potential application not only as a display device, but also for other forms of optical processing such as correlation and holographic switching. Depending on the manner in which it is driven, and the value of the applied voltage, the modulator may be driven at a line rate of at least 10 MHz and a frame rate of up to 15 to 20 kHz, requiring a data input of around 1 to 1.5 Gpixel per second. Typically, while the pixel address time is around 100 nanoseconds, the pixel will actually take around 1 to 5 microseconds to switch between optical states; and while overall frame writing time is of the order of 24 microseconds, the frame to frame writing period is around 80 microseconds.
Our copending International Patent Applications of even filing and priority dates PCT/GB99/04286 and PCT/GB99/04276, refs: P20958WO and P20958WO1, both priority GB9827965.6; PCT/GB99/04232, ref P20959WO, priority GB9327900.3; PCT/GB99/04279, ref P20960WO, priority GB9827901.1; PCT/GB99/04274, ref: P20961WO, priority GB9827964.9; PCT/GB99/04275, ref; P20962WO, priority GB9827945.8; and PCT/GB99/04260 and PCT/GB99/04277, refs: P20963WO and P20963WO1, both priority GB 9327944.1) relate to other inventive aspects associated with this spatial light modulator.
The liquid crystal phase has been recognized since the last century, and there were a few early attempts to utilise liquid crystal materials in light modulators, none of which gave rise to any significant successful commercial use. However, towards the end of the 1960s and in the 1970s, there was a renewed interest in the use of liquid crystal materials in light modulating, with increasing success as maze materials, and purer materials became available, and as technology in general progressed.
Initially, liquid crystal light modulators were in the form of a single cell comprising a layer of liquid crystal material sandwiched between opposed electrode bearing plates, at least one of the plates being transparent. At a later stage, electrooptic nematic devices comprising a plurality of pixels were being devised. Initially, these had the form of a common electrode on one side of a cell and a plurality of individually addressable passive electrodes on the other side of the cell (e.g. as in a seven-segment display), or, for higher numbers of pixels, intersecting passive electrode arrays on either side of the cell, for example row and column electrodes which were scanned. While the latter arrangements provided considerable versatility, there were problems associated with cross-talk between pixels.
The situation was exacerbated when analogue (grey scale) displays were produced by analogue modulation of the applied voltage, since the optical response is non-linearly related to applied voltage. Addressing schemes became relatively complicated, particularly if dc balance (see below) was also required. Such considerations, in association with the relative slowness of switching of nematic cells, have made is difficult to provide real-time video images having a reasonable resolution.
Subsequently, active back-plane devices were produced. These comprise a back plane comprising a plurality of active elements, such as transistors, for energising corresponding pixels, normally for cooperation with a spaced opposed front electrode (counterelectrode) which is often common to all the active elements. Two common forms of active backplane are thin film transistor on silica/glass backplanes, and semiconductor backplanes. In some cases the active elements can be arranged to exercise some form of memory function, in which case addressing of the active element can be accelerated compared to the time needed to address and switch the pixel, easing the problem of displaying at video frame rates.
Active backplanes are commonly provided in an arrangement very similar to a dynamic random access memory (DRAM) or static random access memory (SRAM). At each one of a distributed array of addressable locations, a SRAM type active backplane comprises a memory cell including at least two coupled transistors arranged to have two stable states, so that the cell (and therefore the associated liquid crystal pixel) remains in the last switched state until a later addressing step alters its stale. Each location electrically drives its associated liquid crystal pixel, and is bistable per se, i.e. without the pixel capacitance. Power to drive the pixel to maintain the existing switched state is obtained from busbars which also supply the array of SRAM locations. Addressing is normally performed from peripheral logic via orthogonal sets (for example column and row) addressing lines.
In a DRAM type active backplane, a single active element (transistor) is provided at each location, and forms, together with the capacitance of the associated liquid crystal pixel and counterelectrode, a charge storage cell. Thus in this case, and unlike a SRAM backplane, the liquid crystal pixels are an integral part of the DRAM of the backplane. There is no bistability associated with the location unless the liquid crystal pixel itself is bistable, and this is not usually the case so far as nematic pixels are concerned (although there is one nematic device involving parallel alignment on both substrates with a chiral nematic material, the substrate spacing corresponding to a twist of 90° in the nematic material. In theory this has two stable states according to whether the twist in increased or decreased to accommodate the substrate alignment). Instead, reliance is placed on the active element providing a high impedance when it is not being addressed to prevent leakage of charge from the capacitance, and on periodic refreshing of the DRAM location.
Thin film transistor (TFT) backplanes comprise an array of thin film transistors distributed on a substrate (commonly transparent) over what can be a considerable area, with peripheral logic circuits for addressing the transistors, thereby facilitating the provision of large area pixellated devices which can be directly viewed. Nevertheless, there are problems associated with the yields of the backplanes during manufacture, and the length of the addressing conductors has a slowing effect on the scanning. When provided on a transparent substrate, such as of glass, TFT arrays can actually be located on the front or rear surface of a liquid crystal display device.
In view of their overall size, the area of the TFT array occupied by the transistors, associated conductors and other electrical elements, e.g. capacitors is relatively insignificant. There is therefore no significant disadvantage in employing the SRAM configuration as opposed to the DRAM configuration. This sort of backplane thus overcomes many of the problems associated with slow switching times of liquid crystal pixels.
Generally, the active elements in TFT backplanes are diffusion transistors and the like as opposed to FETS, so that the associated impedances are relatively low in the “OFF” state (so that charge leakage could be significant for DRAM type TFT backplanes).
Semiconductor active backplanes are limited in size to the size of semiconductor substrate available, and are not suited for direct viewing without intervening optics. Nevertheless their very smallness aids speed of addressing of the active elements. This type of backplane commonly comprises FETs, for example MOSFETs or CMOS circuitry, with associated relatively high impedances and relatively low charge leakage in the “OFF” state.
However, the smallness also means that relative to the overall display (array) area the area of the backplane occupied by the transistors, associated conductors and other electrical elements, e.g. capacitors can be relatively significant, particularly in the SRAM type which requires many more elements than the DRAM type. Being opaque to visible light, a semiconductor backplane would provide the rear substrate of a light modulator or display device.
At a later period still, substantial development occurred in the use of smectic liquid crystals. These have potential advantages over nematic phases insofar as their switching speed is markedly greater, and with appropriate surface stabilisation the ferroelectric smectic C phases should provide devices having two stable alignment states, i.e. a memory function.
The thickness of the layer of liquid crystal material in such devices is commonly much smaller than in the corresponding nematic devices, normally being of the order of a few microns at most. In addition to altering the potential switching speed, this increases the unit capacitance of a pixel, easing the function of a DRAM active backplane in retaining a switched state at a pixel until the next address occurs.
However, as the thickness of the liquid crystal approaches the thicknesses associated with the underlying structure of the backplane and/or the magnitude of any possible deformation of the liquid crystal cell structure by flexing or other movement of the substrates, problems arise, for example as to the uniformity of response across the pixel area, and the capability for short circuiting across the cell thickness. The alignment in chiral smectic liquid crystal cells is also frequently very sensitive to mechanical factors, and can be destroyed by mechanical impulses or shock.
It is known to mount electro-optic devices including liquid crystal devices with semiconductor active backplanes in a socket which is thereafter connected to a printed circuit board. The conductors at the edge of the semiconductor substrate are connected, for example by wire bonding, to the conductors of the socket. During the process of mounting the device to the socket, there is the risk of damage thereto, particularly to the semiconductor substrate.
Furthermore, it is not generally possible to guarantee precise physical alignment of the device in the socket, as will normally be required for an optical device, and arrangements for heat sinking are difficult.
European Patent Application No. 96306422.5 (IBM) discloses the lamination of a liquid crystal element which comprises a semiconductor wafer to an optically flat substrate, for example of glass, metal, silicon or a ceramic material. A thermosetting adhesive may be used for the lamination. However, in this case the laminated construction is then adjustably mounted in a holder and external connections are made to the liquid crystal element itself. The substrate appears to have no electrical function such as the provision of bonding pads or tracks for connection to the liquid crystal element of other electrical/electronic components. It is substantially coextensive with the wafer, whereas in the present embodiment a substantially larger hybrid substrate is provided with electrical tracks and pads supported thereon.
U.S. Pat. Nos. 5,543,958 and 5,486,946 (both Motorola) discloses the mounting of a liquid crystal light modulator on a substrate which may be of a ceramic material inter alia, and which bears other components connected to the modulator. There is no disclosure of mounting the substrate on a further printed circuit board, or of the provision of tracks/pads which would make this possible.
It will be appreciated that angular alignment of a relatively small object (e.g. an electro-optic device) involves small displacements of the object edges and so is correspondingly delicate. Alignment of a large object such as a printed circuit board on which the small object is mounted involves a correspondingly large displacement of the board edges, which may be so large as to be unacceptable.